In the design of frames of sports rackets, there are two principal geometrical directions of particular significance according to ball force and string force directions. One is the plane of the string network where the strings are pulling the frame in toward the center of the network, and the other is the plane perpendicular to it which contains the axis of the center of the cross section of the frame. When the string network contains longitudinal strings and transverse strings which are meshing with each other in a weave pattern, each going over and beneath neighboring strings alternately as is required by the prevailing tennis rules, holes in the plane of the network are invariably drilled through the frame to enable the continuous string remaining in the plane of the network to be able to turn around and change its direction. Prior art includes a design in which a steel wire winding spirally around a frame with strings anchored to its inboard side form a conventional string network wherein the string itself is not winding around the frame per se. However, this is a rare exception wherein strings do not project through holes drilled in the mid-plane of the frame. Due to practical difficulties and inconveniences to do otherwise, all sports rackets, especially tennis rackets, have their strings passing through the holes made in the frame at the mid-plane of the frame.
There are design disadvantages, or difficulties, for having the strings wrapping around the frame for support and then proceeding inboardly to form a string network in the plane of symmetry of the frame section in the conventional sense. Since the impact of the ball load is severe, a typical cross section of the sports frame has greater bending strength in the direction of the ball load than in the plane of the string network. Therefore, the height of the cross section of the frame is greater than its width in most cases, whether the section is solid rectangular, I-beam type or hollow tubular. Such height is ideal for having holes in its neutral axis which will not decrease the section's bending strength and which are in the same plane as the string network. But if the string is to wrap around the height of the cross section, the incoming string and the outgoing string will have a large distance between them. To blend the two strings which are at different levels into an interwoven network in the neutral plane would be very inconvenient or difficult to achieve. Another obvious difficulty is the slipping of the string on the frame when the incoming and exiting strings are not perpendicular to the axis of the frame.
Another disadvantage is related to surface cosmetics damage. With the conventional grommet strip running along the middle of the outer periphery of a frame, strings will not touch the frame. The grommet strip can be imbedded in the middle portion of the wall of the frame which results in a smooth belt. The painted surface of the frame is not touched by the string and a good-looking appearance is preserved. If the incoming and outgoing strings are to wrap aound the outer surface of the wall of the section, a curved grommet strip following the contour of the curved section of the frame has to be used and fixed on the wall. The curved grommet strip should follow the wrapping string and provide a separation between the string and the painted surface of the frame. However, this curved grommet strip would be difficult to manufacture accurately and would seem to be unnecessarily complicated in comparison with the conventional straight grommet strip arrangement.
Still another problem facing designers has been the handling of the incoming and outgoing strings which are at two different levels of elevation, both being displaced from the mid-plane of the frame, and to bring these two levels to the mid-plane of the frame as is required. Because of these problems, the prospect of wrapping strings around the frame instead of drilling through the mid-plane to support the string network has been deemed impractical. In all legitimate design wisdom, the conventional string network which anchors all strings through mid-plane holes in the frame has been adopted as the only choice for many years.
The present invention has been devised to resolve all of the foregoing design problems and proposes a frame design in which the strings are supported by the frame: not by holes made in the mid-plane of the frame, but by having the incoming and outgoing strings wrapped around the outboard part of the frame, both for support and for altering direction. The inventive frame has the advantage of simplicity in design, ease in stringing, ease in manufacture (by the injection molding method), economy in production cost (due to injection molding and elimination of the grommet strip), high cross sectional strength and finally, the most important advantage, the elimination of mid-plane holes that lead to premature frame cracking.
The invention proposes a frame whose cross section is a horizontal bar member joined by a vertical bar member. The horizontal bar member is arranged for the incoming and outgoing strings to be wrapped around and provides high bending strength to resist the string tension in the plane of the string network. Since the string tension acting in the plane of the string network pulls the strings inward toward the center of the network, a wide plate-like horizontal member is very effective for providing the bending rigidity the frame requires. The horizontal member is relatively thin so that the incoming and outgoing strings are not spaced far from the mid-plane. In addition, the weaving of the strings in the mid-plane of the network is made relatively easy. The vertical member of the section is high but narrow and is arranged to provide the bending rigidity for the plane perpendicular to the network plane along the ball force direction. The width of the horizontal member and the height of the vertical member are optimally varied along the axis of the frame so that at the head portion of the racket, where the ball force is small but the string tension force is large, the height of the vertical member is reduced and the width of the horizontal member is wide. The height of the vertical member increases toward the throat of the racket so that the increasing moment of the ball's force is more effectively countered.
Small openings are formed on the part of the vertical member at the junction between the vertical member and the horizontal member so as to let strings pass and wrap around the outboard part of the horizontal member and return toward the interior of the network. Unlike the holes in the conventional frame which are highly stressed due to the pull of the string, the small openings only provide passages and low force contact against slipping for the incoming and outgoing strings to wrap around the outboard part of the horizontal member in forming a network. For strings coming at a sharp angle, the vertical member has enough wall thickness to provide supporting surface to prevent sliding of the string. The contact stress between the string and the wall is a compressive stress which the vertical member can easily withstand. Stringing of the racket is a much simpler task since there is no need for conventional grommet strips with holes in the frame.